1. Field of the Invention
The present invention relates to a fuel cell system and a fuel cell control method.
2. Description of the Related Art
A fuel cell is a device that generates electric energy using, in general, hydrogen and oxygen as fuel. Because the fuel cell has a minimal effect on the environment and a high energy efficiency, it has been widely developed as a highly desirable energy supply system.
In the fuel cell, an electrolyte layer made of a solid polymer is interposed between an anode and a cathode to form a membrane electrode assembly (MEA), and the MEA is held between separators.
In the fuel cell, a hydrogen-containing gas is supplied to the anode and the hydrogen is dissociated over a catalyst into protons and electrons. The protons move toward the cathode through the electrolyte layer. The electrons produced by the dissociation of the hydrogen are taken out to an external circuit to be utilized as DC electric energy. Oxidant gas that contains oxygen is supplied to the cathode. At the cathode, the protons, the electrons, and the oxygen react to generate water.
If the amount of water contained in the solid polymer electrolyte is reduced, the ion conductivity of the solid polymer electrolyte decreases. Thus, it is necessary to maintain the water content of the solid polymer electrolyte at a predetermined level in order to continue power generation. In view of the above, Japanese Patent Application Publication No. 2002-175821 (JP-A-2002-175821) describes a process of controlling the back pressure of oxidant gas according to the internal resistance of the fuel cell, the relative humidity of off gas, etc., to maintain the water content of the electrolyte membrane at a predetermined level.
In order to improve the power generation efficiency of the fuel cell, it is preferable not to hydrate the electrolyte membrane as long as the power generation performance of the cell may be quickly recovered. However, the technique described in JP-A-2002-175821 does not take into account the conditions under which the power generation performance may be quickly recovered.